posted on January 30, 2012 08:30
Turbo Tech: Compressor and Turbine Map Details
By Khiem Dinh
Khiem Dinh is an engineer for Honeywell Turbo Technologies at the time of this writing. All statements and opinions expressed by Khiem Dinh are solely those of Khiem Dinh and not reflective of Honeywell Turbo Technologies.
We have previously covered compressor wheel technology and also how compressor and turbine maps are generated. Now we are going to dive a little deeper into the details of the compressor map and all-mysterious turbine map. Furthermore, we are going to see how compressor wheel and turbine housing A/R changes affect turbine flow and efficiency.
The basic compressor map plots air flow and pressure ratio for a given compressor speed. On the vertical axis is pressure ratio which is the compressor outlet pressure divided by the compressor inlet pressure. We have mentioned this before, but what we neglected to mention was the type of pressure being measured; for compressor maps, we need to compare total pressures at the inlet and outlet of the compressor housing. The total pressure of a flow is segregated into ambient, static and dynamic pressures. We need to measure the total pressure due to different size piping changing velocities and therefore the dynamic and static pressures.
|The pressure ratio calculated on compressor maps is generally a total-to-total pressure taking into account the ambient, static, and dynamic pressures. We need to use total pressure due to Bernoulli’s principle that the total pressure is constant along a streamline. Looking at the above diagram, let’s say that Area #1 is the compressor outlet and Area #2 is the larger diameter tube after the compressor outlet. The dynamic pressure is directly affected by the velocity of the air and that is a function of the area of the tube it is flowing through. Smaller area = higher velocity and vice versa. Because the areas of #1 and #2 are different, the velocities are different therefore making the dynamic pressures different. Because the dynamic pressures are different, the static pressures (the ones most often measured) are also different even though the total pressures are the same at #1 and #2. So if only the static pressure is measured, the incorrect pressure ratio may be calculated.
Compressor maps also show a corrected mass flow and corrected compressor speed. While the maps are generated in a controlled environment, the conditions will not be exactly the same every time. Therefore, the mass flow rates and compressor speeds are corrected based on the environmental ambient pressure and temperature during testing.
|Mass flow rate (Q) is corrected to reference values of temperature and pressure, much like how many chemistry calculations are done at STP(standard temperature and pressure). By correcting every test to the same reference temperature and pressure allows the maps to be confidently compared. The speed of the compressor (N) also needs to be corrected.
The turbine map is orientated differently than the compressor map with the pressure/expansion ratio on the horizontal axis and the corrected flow on the vertical axis; turbines are also known as expanders as the gas starts off at a high pressure entering the turbine and expand to a lower pressure as it exits the turbine. Therefore, when speaking specifically about turbines, the pressure ratio is also known as the expansion ratio. Back on topic to the turbine map, turbine efficiency also looks different as compared to the compressor map; there are efficiency lines which correspond to each compressor speed line with the efficiency values also plotted on the vertical axis.
|Here is a real deal turbine map from Borg Warner. The top set of red curves are the efficiency lines and the bottom set of green lines are the flow rates each labeled with their corresponding compressor speeds. The turbine map also has corrected mass flow rates and corrected speeds just like the compressor map. However, one difference is how the turbine pressure ratio is calculated; the pressure ratio on the turbine side uses total pressure at the turbine inlet and static pressure at the turbine outlet.
Monday, January 30, 2012 8:25 AM
Always a good read, thanks. I just wish manufacturers would release turbine maps of their more popular turbine/compressor combos.
Monday, January 30, 2012 8:33 AM
Agree, though we're lucky we even get compressor/turbine maps.
Monday, January 30, 2012 6:46 PM
Good read! More turbo knowledge is a good thing.
Tuesday, January 31, 2012 12:35 AM
Spread the word!
Wednesday, February 01, 2012 2:04 PM
Ah. Wakarimashita. Arigato gozaimasu! ^_^
Saturday, October 20, 2012 2:31 PM
@spdracerut, I still don't understand where the operating line falls on these compressor maps.
The operating line defines the match between the compressor characteristic and turbine's characteristic. Probably different engines feeding the turbo will have different operating lines due to different compression ratios, VE, etc. but understanding how the engine and turbos operate both steady state and transiently is critically important in optimizing the system. Any ideas how this works? I've seen BW and Garret have some generic apps available off their websites which are probably pretty good but I don't know how they work. Do you?
Monday, October 22, 2012 7:40 PM
The Borg Warner Matchbot is more accurate in that it ties in the turbine map/pressure ratio. However, it requires very exact inputs of data for which the general consumer does not have: BSFC, volumetric efficiency, intercooler pressure drop, exhaust restriction, air filter pressure drop, and turbine efficiency.
If you don't have that data, you're just going to have a ballpark estimation. The Turbo by Garrett Boost Adviser gives you a quick and dirty ballpark estimate as to which turbo size you should be using. If you don't have detailed info of the parameters I mentioned, then the Garrett Boost Adviser essentially gives you the same result while being much easier to use. And then there's this wondering thing called the internet where someone has probably run the turbo you're looking at on your same application; so that can help you narrow turbine housing size selection to get the spool and power characteristics you want.
Monday, October 22, 2012 7:43 PM
Also, keep in mind your BSFC is related to your exhaust backpressure, which is also a function of your turbine housing sizing. So..... I just use the quick and dirty because I don't usually have access to super detailed engine cell type data.
Monday, October 22, 2012 9:43 PM
@spdracerut, thanks for your response. Your point that we won't know exactly the conditions that the engine is running without an expensive engine dyno makes sense, so probably the Garret or Borg Warner apps are close enough. That may be so but without seeing a simple sensitivity study like you did in "Generating Compressor and Turbine Maps" I'll never really know. I have no sense for the relationship between BSFC, Dpqp, VE, etc. based on idealized theory or empirical data and I haven't found anyone that does... yet. Do you know where I can find this stuff to at least convince myself that the turbo apps really are close enough, close enough to clearly discriminate the advantages and disadvantages of the small number of turbos available? BTW, the Garret web based one craps out at the point where you have to enter either the zip code or hard code the atmospheric conditions.
I much appreciate the articles you've put together out here.
Tuesday, October 23, 2012 12:44 AM
Well, there's a difference between sizing for an OEM type of application and just your typical street or aftermarket car getting modded for more power.
In an OEM application, they have very specific goals and also should be able to relay estimated pressure drops and exhaust restriction values. So you can adjust wheel trims, turbine A/Rs, etc.
For you typical aftermarket consumer, they change the exhaust, intake, etc and all those values along with volumetric efficiency go out the window. VE changes with cam profiles too. And BSFC changes with all that along with depending on cylinder head design, etc etc.
So I just use some basic rules of thumb for sizing turbos. First you have to know what type of fuel you'll be using. Race gas/E85/93 octane with meth, and a turbo will do roughly 9whp (dynojet) per lb/min of airflow. Using 91 octane, it's about 7whp per lb/min of airflow. These numbers assume you have all the supporting mods such as cams, exhaust, intake, etc. So pick the smallest compressor wheel that will do the job. There tends to a bit of overlap for turbine wheel sizes for the same compressor wheel. So this takes a bit of feel, but the bigger the displacement the engine, the bigger the turbine wheel. So the last tweak is picking the turbine housing A/R. This comes down to if you want faster spool/less power or vice versa. And this is where finding someone with a similar setup helps :)
I tend to do just the rule-of-thumb for this reason: take a 300whp Evo and put on a test pipe and exhaust and you can uncork 30whp. That's a 10% difference which is about the same magnitude of change as a step size in A/R turbine housing. Therefore, the Garrett app is close enough IMO. I checked the web app and it is messed up, but the Android app works fine.
Tuesday, October 23, 2012 12:54 AM
Oh, the Boost Advisor is good for a quick check to see where you are on the compressor map to see how close to surge or choke you are.
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